Why do I see mining bees in September?

In the past week, I received two nearly identical questions about mining bees. Both writers wanted to know why they had mining bees in September while everyone says that mining bees appear in the spring. Excellent questions.

DJ wrote:

All I read about these bees indicates they are usually around in early spring, but mine are here now and intermittently throughout the summer. Is that normal? I have a lot of flowering plants like catmint which all of my bee friends seem to love and I’m wondering if they are just visiting me longer because of all the pollen and goodies. I feel bad they have landed where they did because it is near where we walk for our hose and I am always walking on their little mounds of sand and soil.

Shortly afterward, Keith wrote:

I was never aware of mining bees until this past week when literally thousands of them were hovering over a bare area on the property. My question is most comments seem to refer to spring, and a relatively short span of activity. But this is early September! Does timing of activity vary that much with species, or is something else going on? I hadn’t seen the myriad of individual, small holes in the ground until I got down on my knees to look at/try to identify the fliers. From a standing position, it was impossible to see what they were because of their small size and rapid movement.

I love that these two people have enough confidence in their observations to question what they’ve heard about mining bees in September. And of course, they are both right. The species that are active change throughout the year, but some species will be around as long as flowers are blooming.

Are they native, wild, or feral?

Any discussion about bees has to begin with the words we use to describe them. There doesn’t seem to be a standard, but you can categorize bees by how they got here.

For example, most of the bees here in North America are native to the continent and were here when the colonists arrived. But others have been introduced on purpose and subsequently escaped into the environment. These escapees became established, much like introduced weeds. Examples are wild populations of the European honey bee and the alfalfa leafcutting bee.

Still, other bees were introduced accidentally and were never managed, such as the European wool carder bee. All three of these categories can be referred to as “wild.” The opposite of wild bees includes those that are currently managed, such as most European honey bees.

Common bee names are confusing

When it comes to identifying bees, I again think it’s the names that confuse people. For example, many people use the term “mason” bee to refer to bees in the genus Osmia, but in a larger sense, mason bees are any bees that collect building materials from their environment. So that makes leafcutting bees, wool carder bees, and pebble bees masons as well.

Likewise, the terms “mining bee,” “digger bee,” and “sweat bee” each comprise hundreds and sometimes thousands of different species. So when one person describes what he considers a mining bee, it probably isn’t the same species that someone else is calling a mining bee. These names are based on broad descriptions of behavior; they are not names of specific species.

Sometimes there is a bit of correlation that develops over time. For example, the term “mining bee” is often used to describe bees in the genus Andrena, while “digger bee” is used for the genus Anthophora, but both are large groups of bees that dig small holes in the ground. The genus Andrena alone comprises 1300 species.

What people need to realize is that Earth is home to roughly 20,000 named species of bee and probably another 10,000 to 20,000 unnamed ones. So when you use a label like “digger bee” you have to understand that the word encompasses a wide range of bees and bee behavior.

Mining bees appear in different seasons

Going back to the original questions, both DJ and Keith mention hearing that mining bees appear in the spring, but their mining bees are busy in September and intermittently during summer. All I can say is they are quite observant.

In truth, different species of bees become active at different times of the year. A species can become active in late winter, early spring, mid-spring, late spring, early summer, mid-summer, late summer, early fall, and so on. Each species is different and each one has a specific time when it becomes active. Its active time usually correlates with the flowering plants it prefers. When those flowers bloom, the bees “bloom” as well.

Unlike honey bees which do not hibernate, most bees spend the majority of their lives in the nest either as a larva or a pupa. The time spent in the nest varies with species, but as a rule of thumb, you can imagine bees spending about 10 months in the nest and two months as active adults.

When we say bees “become active” we mean the adults emerge, mate, lay eggs, and gather provisions for their young. Once this work is done, the adults die and the next generation is on hold for 10 months until next year.

Species come and go throughout the year

So when DJ says her bees are active intermittently, what she is seeing is different species coming into their active period at different times of the year. Of course, nature is complex and nothing is easy to pigeonhole.

For example, there are species of bees that have two life cycles in one year. Other bees—like some of the bumbles—appear to have longer active periods because they live in colonies that persist for months. And the real outliers, colonies of honey bees, do not have a dormant or hibernating stage at all. But for most species, the life cycle is less than 2 months of activity followed by 10 months holed up in the nest in an immature form.

Without actual specimens, it’s impossible to say if the mining bees DJ saw in September are the same species she saw in spring. Maybe yes, maybe no. But in any case, they would not likely be the same individuals.

Because bees become active along with the flowers they prefer, the mix of bees changes with the mix of flowers. That means if you were to inventory all the bees in your backyard on May 1 you would have a very different list than you would on a different date, let’s say August 1. Honey bees would be on both lists, and maybe some of the bumble bees, but the others would be different. Even if you were to compare inventories one month apart, say May 1 and June 1, you would see a big difference. Just as the flowers change, so do the bees.

Spring brings lots of variety

I agree that people say mining bees are active in the spring and then disappear, and I’m sure I’ve said that too. The reason, I think, is that many more species are active in the spring than in the fall, just as many more flowers bloom in the spring. But that doesn’t mean there are no mining bees the rest of the year. Bees are all over the place, and many of them find flowers the rest of us barely notice.

The same applies to mason bees. When people say masons are only active in the spring, they are talking about specific species, maybe Osmia lignaria or Osmia aglaia. But there are also very early masons, late spring masons, summer masons, and so on.

DJ mentions the number of flowering plants she has and wonders if that is a cause for all the bees. Absolutely! Remember bees and flowers are dependent on each other, and where you find a lot of one, you will find a lot of the other. The more flowering plants you have, the more bees you will have, and not just in numbers but also in variety. That is why flowers are so important to bee conservation: more varieties can support more species of bees.

Walking on bee mounds

DJ’s last question is about walking on the mounds, technically known as tumuli. If you go out of your way to destroy all the tumuli all of the time, you will do damage to your bee population. On the other hand, If you accidentally step on a few now and then, the bees will quickly repair the damage. Like many things in nature, it’s a matter of degree.

I recommend you use your yard normally and not worry about the mounds. I’ve seen soil-nesting bees in playgrounds, ballparks, picnic areas, backyards, and parking lots. They survive just fine as long as we don’t go out of our way to destroy them.

The holes come in many sizes

Keith mentions the holes in his yard are very tiny, while others claim they are quite large. But once again, the size of the hole is species-dependent. Of the 20,000+ species of bee in the world, fully 70% live in holes in the ground. That means there is an enormous variation in size, density, depth, seasonality, and soil types involved. The world of bees is anything but simple.

I don’t know if I clarified anything or not, but I love hearing from people who care. Most people ask how to kill mining bees, even in September, so it’s a pleasure to hear from the others.

Rusty
Honey Bee Suite

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